The present invention is in the field of paper manufacturing. More specifically, the invention in one embodiment is directed towards a process for preparing a paper web, such as a sheet of newsprint, from low-grade paper furnishes. In another embodiment, the invention is directed toward a process for preparing a paper web from pulp that contains coated broke. The invention further is directed towards a paper web prepared in accordance with the inventive processes.
Conventionally, paper is made by extracting a paper web from a slurry of wood pulp. In accordance with conventional papermaking processes, a furnish composed of wood pulp is provided and is introduced into a paper making machine. To the furnish are added various additives, the additives selected to affect the properties of the paper and the paper machine runnability. At the headbox of the paper making machine, pulp slurry from the headbox is deposited on a wire, and water is removed, thus forming a paper web from the slurry. The web is dried, optionally is collected on a reel, and subsequently is cut into sheets or otherwise converted for its intended application.
Generally speaking, paper may be classified into various grades, with higher grades of paper being known as xe2x80x9cfinexe2x80x9d papers, and with many lower grades also being known. The grade of paper is largely determined by the quality of the pulp in the paper furnish. One form of lower grade paper is newsprint, the paper stock that is used to print newspapers. Enormous quantities of newsprint are consumed daily by newspaper printers. Because of these high volumes and the short life expectancy of printed newspapers, printers and manufacturers of newsprint find it especially important to minimize the costs of newsprint stock, with cost generally being of somewhat more concern than paper quality. For this reason, newsprint furnishes typically contain lower grades of wood pulp than are found in furnishes that are used for the production of fine papers. For example, newsprint stock typically contains large amounts of groundwood stock and/or recycled paper stock. Typically, at least 30% of the pulp found in newsprint furnishes is composed of groundwood stock and/or recycled stock, and in some instances, as much as 100% of the pulp in the newsprint furnish is so composed.
Both groundwood stock and recycled paper stock typically contain large amounts of impurities. Such impurities can adversely affect the quality of paper produced from such furnishes. For example, groundwood stock is prepared via the high-yield mechanical processing of raw wood. Such stock typically contains relatively large amounts of components that are undesirable in the manufacture of paper, including such components as lignin, residue, pitch, resins, carbohydrates, fatty acids, and fiber fines. All of these components are believed to adversely affect various properties of the paper produced from groundwoods, such as strength, brightness, color, opacity, smoothness, and printability.
Retention aids also are used in connection with the manufacture of fine papers. Recycled stock and groundwood stock are sometimes used in the manufacture of fine papers in relatively smaller amounts than in newsprint. In the manufacture of such papers, the prior art teaches that the brightness, opacity, and other properties of the paper sheet may be improved by incorporating a filler into the paper furnish prior to forming a paper web therefrom. In conjunction with the preparation of such fine papers, fillers are believed to enhance opacity, brightness, color, smoothness, and printability properties. In addition, fillers may be used to provide improved machine runnability and a reduction in sheet cost. Conventional fillers are composed of fine particles, such as clays, which are not easily retained as the paper web is being formed from the pulp slurry. Accordingly, the prior art teaches to add a retention aid to the slurry to assist in retaining the filler material within the paper web. The use of such fillers in connection with retention aids has been satisfactory in connection with the manufacture of fine papers.
One source of recycled paper stock is xe2x80x9cbrokexe2x80x9d stock, which typically is stock that failed to meet the particular specification for a given application and which therefore cannot be sold. Often, broke paper is coated with a variety of coatings, in which case it is termed xe2x80x9ccoated broke.xe2x80x9d Coatings typical of coated broke include pigments and adhesives which are intended to improve the properties of the product. Pigments, which are used in coating color formulations, usually are the main coatings, and generally provide between 80 to 95 percent of the total coating mass. Pigments include various forms of clay, calcium carbonate-both precipitated and ground, and titanium dioxide, among others. Adhesive materials generally are found in lower quantities, typically, between 8 to 20 percent of the total coating mass, and can include various modifications of starch and latex polymers such as styrene-butadiene, polyacrylics and polyacrylates. These components can adversely affect the properties of paper produced from coated broke. Recycled paper stock can contain other undesired components, including fine fibers, other chemical residues from prior paper processing, and possibly other undesired components. Moreover, because the quality and composition of recycled paper stock may vary from one source to another, the composition of recycled paper stock may be largely unknown. Accordingly, when recycled stock is provided in the furnish, a number of difficulties in maintaining the quality of paper prepared from such furnishes are encountered.
One of the problems associated with the production of paper from recycled paper stock, especially stock containing coated broke, is the accumulation of adhesive material in the wet end of a papermaking system. The accumulations known as xe2x80x9cwhite pitchxe2x80x9d form blemishes in the finished sheet, reducing its quality and often causing it to fail to meet desired specifications. White pitch also causes frequent web breaks in production runs, causes felt and wire clogging, and limits water drainage, all of which may result in decreased machine speed and increased machine downtime.
The prior art teaches that these difficulties with impurities can be addressed by adding retention aids, such as polyDADMAC, acrylamides, and epichlorohydrin/dimethylamine, to the pulp material. Retention aids cause the problematic substances to bind to the pulp fiber and be removed from the system as the fibers are retained in the newly formed paper web. However, the usefulness of this approach can be limited by the tendency of the treated pulp material to over-flocculate. Over-flocculation reduces machine performance by reducing the efficiency of sheet dewatering, with the result that physical sheet properties including formation, strength, and optical properties decrease. Use of retention aids also can thus be limited in maximizing the retention of broke material, thereby limiting the usefulness of the retention aid in mitigating against white pitch formation.
In recognition of these problems, the prior art has presented a number of proposed solutions. For example, U.S. Pat. No. 4,997,523 (Pease et al.) purports to disclose a method for treating coated broke using cationic and anionic coagulants. Another patent, U.S. Pat. No. 5,131,982 (St. John) purports to disclose treatment of coated broke using polyDADMAC. A third patent, U.S. Pat. No. 5,466,338, purports to disclose the use of a xe2x80x9cwater-soluble dispersive polymerxe2x80x9d to treat repulped broke. These proposed solutions are not always satisfactory. These polymers can have a tendency to cause overflocculation of the sheet as it is being formed, leading to reduced properties including formation, strength, and optical properties.
Another limitation with the use of retention aids arises in the manufacture of newsprint and similar paper webs from lower-grade furnishes. In the manufacture of such webs, it is more difficult, and sometimes impractical to use a retention aid to incorporate a filler into the paper product. Because relatively large amounts of lower grade pulps are present in newsprint furnishes, impurities and other undesired components associated with such pulps are present in relatively large amounts. Thus, when attempts are made to incorporate a filler into, for example, a newsprint web, relatively large amounts of impurities are retained in the paper web along with the filler particles. The retention of such undesired components along with the filler particles largely or completely offsets any benefits obtained by the incorporation of the filler into the paper web.
For these reasons, a need exists for a process for preparing a paper web from low-grade newsprint pulps while incorporating a filler into the paper web. The process should avoid the drawbacks associated with conventional methods of incorporating a filler into a paper web. In some aspects of the invention, it is a general object to provide a process for preparing a paper web from lower-grade furnishes that include groundwood pulp, recycled pulp, or mixtures thereof, and for incorporating a filler into the paper web without also incorporating into the web amounts of impurities in the newsprint furnish that are sufficient to offset the benefits of incorporating the filler into the web. Another general object of these aspects of the invention is to efficiently retain a filler in a newsprint web while maintaining the beneficial properties of the filler.
A further need exists for a process for preparing paper from recycled pulps that contain coated broke. The method should alleviate the white pitch problem associated with the use of coated broke, and should avoid over-flocculation that interferes with efficient paper production and paper quality. It is a general object of this aspect of the invention to provide a process for producing paper from recycled pulps that reduces or eliminates the white pitch problem.
It has now been found that newsprint may be prepared from lower-grade pulp furnishes by adding a pre-flocculated filler to the furnish prior to forming a paper web therefrom. Surprisingly, it has also been found that, when filler is incorporated via the addition of a pre-flocculated filler, rather than by adding conventional filler and subsequently adding a retention aid, impurities such as lignin, pitch, ink particles, and other impurities commonly found in newsprint furnishes will not be incorporated into the paper web to the same degree as with the conventional process. Even more surprisingly, many properties of a newsprint sheet prepared from such furnishes are substantially improved as compared with those of conventionally prepared newsprint sheets.
Thus, the process of the invention includes the steps of providing a newsprint furnish, adding a pre-flocculated filler to the furnish, and forming a paper web from the furnish. Typically, the web will be dried, printed with newspaper ink and subsequently cut into sheets.
In accordance with a particularly preferred embodiment of the present invention, the pre-flocculated filler is prepared in accordance with the teachings of U.S. Pat. No. 4,799,964, issued to Richard D. Harvey et al. and assigned to Grain Processing Corporation of Muscatine, Iowa, which is incorporated herein by reference in its entirety. The process for forming a pre-flocculated filler taught in the foregoing patent comprises continuously bringing together an aqueous slurry of a paper filler material and a flocculating agent, and imparting to the mixture a shearing force sufficient to provide a flocculated filler of controlled particle size. When a pre-flocculated filler is prepared in accordance with the preferred embodiment of the invention and added to the newsprint furnish, a newsprint of high quality may be economically prepared.
It has also been found that paper formation from furnishes containing recycled broke pulp is facilitated by adding a flocculating chemical to the recycled pulp before the recycled pulp is added to a paper furnish, and by applying a sufficient shearing force to the flocculated pulp to reduce or maintain the floccs to a size that is effective to enhance the retention of floccs in a paper web. Surprisingly, this process not only reduces white pitch problems, but also improves paper qualities including such properties as formation strength and opacity, as compared to paper prepared from conventional recycled coated broke.
The invention also encompasses paper webs made in accordance with the foregoing processes.